This invention relates to packaging electronic devices such as integrated circuits (ICs).
Integrated circuits are normally fabricated on a silicon wafer. The wafer may carry multiple separate ICs. The wafer is then cut up so that each IC is on an individual die, and the dies are then packaged for protection inside an insulating housing which has external contacts by which electrical connections can be made to the IC.
FIG. 1 shows partial cross-sections of four types of IC package. Like components are designated with the same reference numerals in each type of package.
Package 1 is a quad flat pack (QFP) package. The IC die 2 is mounted to a die attach pad 3 by adhesive 4. Leads 5 are attached to the die by wires 6 and the whole assembly is encapsulated in a block 7 of an insulating material. In the QFP package the leads 5 extend outside the footprint of the block 7 of insulating material.
Package 8 is a quad flat no-lead (QFN) package. In this package the leads 5 are flush with the lower surface of the block 7 and lie within the footprint of the block. Since the leads do not extend outside the footprint of the block, this yields a smaller package than the QFP technique.
Package 9 is a chip on lead (COL) package. In this package the leads extend under the die and so the leads, rather than a die attach pad, support the die. The leads are still connected to the die by wires 6.
Package 10 is a flip-chip on lead (FOL) package. In this package the leads are coupled directly to the die by means of solder balls 11, without the need for wires.
Finally, package 12 is a ball grid array (BGA) package. The die is coupled to a circuit board 13 which is packaged in the block 7 along with the die. The circuit board contains tracks 14 which terminate in exposed pads within the footprint of the block. In packages 1, 8, 9 and 10 the leads 5 terminate around the periphery of the package, whereas the BGA arrangement allows the pads 14 to be configured in an array over the lower surface of the block 7.
All these packaging arrangements suffer from disadvantages. The QFP package is relatively large because the leads extend outside the block 7. In the QFN, COL and FCOL packages the external contacts are arranged in a row around the periphery of the package, which restricts the number of external contacts that can be provided if connections are to be made reliably to the contacts. The BGA package allows the external contacts to be spread over the area of the package but it requires the circuit board 13, which increases its cost.
There is therefore a need for an improved design of package for electronic devices.